The present invention relates to a secondary storage subsystem including a secondary storage controller and a secondary storage in which the secondary storage subsystem is connected to a host such as a central processing unit (CPU) to store therein data necessary for,the host and to restore the data when required. More specifically, the present invention relates to an array disk subsystem having a plurality of secondary Storage controllers and a plurality of secondary storages wherein the subsystem is capable of achieving ordinary input/output processing even while a backup processing is being executed. The backup processing here is accomplished to store information of the secondary storage in another store means for a backup use, which is effective for an acquisition of data when an access to the data stored in the subsystem is prevented due to a failure such as an erroneous operation conducted by an operator, a software error, or a hardware error.
In computer systems at the present stage of art, data necessary for the host is loaded in a secondary storage such that the data is obtained from the secondary storage when a data request is requested from the host. As the secondary storage, a nonvolatile storage is employed in general; typically, a direct access storage device such as a magnetic disk or an optical disk is adopted. Although these secondary storage devices have a considerably high reliability, there may still exist a chance of a failure which prevents data stored therein from being accessed.
In order to cope with the difficulty above, there has already been known a backup processing method in which, as shown in FIG. 6, during a period of time when a computer system is not utilized from an external device, data stored in a magnetic disk 11 of a secondary storage is transferred via a disk controller (DKC) 14 to a CPU 1, which then records the data on a magnetic tape (MT) 12 via a magnetic tape controller (MTC) 15 or on an optical disk (OD) 13 via an optical disk controller (ODC) 16.
Furthermore, in accordance with the present invention, there is accomplished a parallel processing in which a plurality of disk drives each being associated with a relatively small capacity are prepared so that data transferred from the CPU 1 is split into subdata items to be stored in the respective disk drives. A parallel processing system of this type has already been put to the market, for example, from Storage Concepts Corp. A method of improving the reliability in such a system has been described in "Providing Fault Tolerance In Parallel Secondary Storage Systems" written by Arvin Park and K. Balasubramanian of the Department of Computer Science, Princeton University (Nov. 7, 1986). The configuration of the system has also been discussed in the report above.